The invention relates to a process for the preparation of polyether polyols by means of double metal cyanide (DMC) catalysis, in which the induction phase is shortened markedly.
According to prior art polyether polyols are prepared by polyaddition of alkylene oxides to starter compounds having active hydrogen atoms, by means of metal hydroxide catalysis (for example KOH), see Ullmanns xe2x80x9cEncyclopxc3xa4die der technischen Chemiexe2x80x9d [Encyclopaedia of Industrial Chemistry], Vol. 14, 1963, p. 49 et seq. The polyaddition reaction rates are very slow in this process. Depending on the reaction temperature, catalyst concentration and OH number of the polyether polyol prepared, in the prior art process monofunctional polyethers having terminal double bonds, so-called monools, form additionally and restrict the use of the product for subsequent polyurethane applications. Furthermore, after the polyaddition the base which was used must be removed. This can be achieved, for example, either by the addition of acids, by the use of neutralising adsorbers, ion exchangers or by other methods. The water of neutralisation which arises in this case and the salts which have formed must likewise be separated before further processing. This brief description of the process used according to the present state of the art illustrates the labour- and cost-intensive nature of polyether polyol preparation.
Double metal cyanide (DMC) catalysts for the preparation of polyether polyols have long been known (see, for example, U.S. Pat. Nos. 3,404,109, 3,829,505, 3,941,849 and 5,158,922). The use of these DMC catalysts for the preparation of polyether polyols in particular brings about a reduction in the proportion of monofunctional polyethers having terminal double bonds, so-called monools, by comparison with the conventional preparation of polyether polyols by means of basic catalysts. The polyether polyols thus obtained can be processed to high-grade polyurethanes (for example elastomers, foams, coatings). Improved DMC catalysts such as are described in EP-A 700 949, EP-A 761 708, WO 97/40086, WO 98/16310, DE-A 197 45 120, DE-A 197 57 574 and DE-A 198 10 269, for example, are additionally extremely highly active and enable polyether polyols to be prepared at very low catalyst concentrations (25 ppm or less), such that it is no longer necessary to separate the catalyst from the polyol.
Induction times which are sometimes protracted are a disadvantage of DMC-catalysed polyether polyol preparation. During this phase the chain is built up only very slowly, such that only a very poor space-time yield is achieved. This in turn means a reduction in the economic advantage afforded over the KOH-catalysed process by the more rapid polyaddition and substantially simplified working-up of the product.
One possibility of reducing the induction times resides in elevating the alkylene oxide concentration in the reactor. However, a high concentration of free alkylene oxide constitutes a major hazard potential, because catalyst activation may result in intensified liberation of heat and consequently uncontrolled temperature increases. This would lead to greater thermal loading of the polyol, such that on the one hand, product quality might be impaired, for example owing to higher viscosities or broader molecular weight distributions and, on the other hand, the catalyst activity could be reduced by accelerated ageing. In an extreme case, an uncontrolled temperature increase may even lead to adiabatic thermal decomposition of the polyether polyol.
In the light of the difficulties described in the section above, the reactor is normally (see, for example, WO 97/23544) charged only with part of the total quantity of alkylene oxide necessary for carrying out the reaction to the polyol. Then only after waiting until a pronounced pressure drop in the reactor signals that the catalyst is completely activated, is further alkylene oxide supplied to the reactor.
It has now surprisingly been found that the induction phase in DMC-catalysed polyether polyol preparation can be shortened markedly if small quantities of alkylene oxide are supplied in continuous manner during the induction phase also, preferably quantities such that a constant pressure prevails in the reactor. The quantity of free alkylene oxide used for activation should here be oriented on the basis of either the maximum permissible reactor temperature (Tmax) or, if this is above the decomposition temperature of the polyether polyol to be prepared, on the basis of the decomposition temperature (Tmax) thereof. The threshold concentration (Cthreshold) of free alkylene oxide in the reactor can be calculated as a function of the reaction temperature (Treaction) and the reaction enthalpy (xcex94HR), in accordance with the following formula:
Cthresholdxe2x89xa6Tmaxxe2x88x92Treaction/xcex94HR